The present invention relates to a method for preparing an oriented and textured material.
It applies especially to the preparation of magnetic materials designed to form "soft" or "hard" magnets or high temperature superconductors.
More particularly, the invention provides a method for preparing in a vessel an oriented and textured magnetic material using, in combination:
an orientation effect, caused by a magnetic field, of the seeds or crystallites of the material it is desired to manufacture, this material being in a molten state or dispersed in a molten compound;
a sedimentation effect caused by a magnetic force in order that, during their formation, the seeds or crystallites of the desired material gather in a same area of the vessel, generally at the bottom, causing a purification action since the various parasitic particles liable to exist in the molten mass or in the dispersion and which present a magnetic susceptibility different from that of the desired material are not attracted with the same efficiency as the desired material; and
a texturation effect associated with the creation of a temperature gradient in the area where the sedimentation occurs, in order to improve an aggregation or solidification according to the growth axes of the desired material, to obtain it as a single crystal or oriented crystallites.
Before explaining in more detail the invention, a few general magnetization laws, the invention makes use of, will be reminded.
First, magnetic materials have a magnetic susceptibility X which is generally anisotropic. For example, there are materials that have an axis of easy magnetization, hereinafter called axis c, the two other axes being axes a and b. Thus, if X is the magnetic susceptibility, the difference in magnetic susceptibility between the axis of easy magnetization (c) and the hard directions (a and b), is: EQU .DELTA.X=X.sub.c -X.sub.ab
If a magnetic field B is applied, particles tend to be oriented according to their axis of easy magnetization and an energy gain .E is produced with respect to the case of a material with a random distribution of the magnetic axes: EQU .DELTA.E=V.B.sup.2..DELTA.X/2 .mu..sub.0
where V is the volume considered and .mu..sub.0 =4.pi..10.sup.-7 in international units (I.U.).
If it is desired to orientate a magnetic material in a field, this energy gain .DELTA.E must be substantially higher than the energy associated with the thermal agitation, namely, kT, where T is the absolute temperature and k the Boltzmann's constant.
The result of this comparison gives the definition of volumes or elementary domains liable to be satisfactorily oriented. For example, for a YBa.sub.2 Cu.sub.3 O.sub.7 grain of 1 .mu.m.sup.3, which constitutes a high temperature superconductor, .DELTA.X will be about 10.sup.-5 I.U. which gives .DELTA.E/kT=10.sup.4 at T=1500.degree. K. and for B=5 teslas, that is, .DELTA.E &gt;kT. But, .DELTA.E/kT is equal to 10 only if the grain size decreases to 10.sup.-3 .mu.m.sup.3.
The simplest case of uniaxial anisotropy will be considered here. However, it is known that some magnetic materials may have several equivalent axes of easy magnetization and even an easy magnetization plane. This magnetic anisotropy may be very high when the material is magnetically ordered, particularly when it is ferromagnetic. In the paramagnetic state, the magnetic anisotropy is very low but often sufficient for alignment under a magnetic field.
On the other hand, when considering the magnetic force applied to a material in the case of an induction B with a gradient dB/dz, the product B.dB/dz being about 500 T.sup.2 /m, it can be demonstrated that a rare earth (R) compound of the RBa.sub.2 Cu.sub.3 O.sub.7 type, will be subjected at 1500.degree. K. to a force of about 7 times gravity if R is dysprosium or erbium, and about 0.5 time gravity if R is neodymium. A compound of the Nd.sub.2 Fe.sub.14 B type will be subjected at 1500.degree. K. to a force equal to 30 times gravity and, at the eutectic solidification temperature, at about 1000.degree. K., to a force equal to 50 times gravity. These orders of magnitude show that the sedimentation effects associated with the presence of a magnetic force on a magnetic material ensure performances substantially equal to those obtained with centrifuging techniques.
The principles reminded above are intended to call back to mind the orientation effect that can be obtained by the application of a magnetic field, and the sedimentation effects that may result from the application of a magnetic force.
Moreover, it is known that, in order to facilitate the solidification of a material according to its preferential growth axis or plane, it is desirable to apply, during cooling down, a temperature gradient in the direction of this growth axis or plane. In practice, this means that, in case of a material under solidification placed in a vessel, and sedimenting on the lower portion of this vessel, the bottom or the walls of this vessel during solidification are preferably cooled down.